Disc bits and other downhole tools that use disc cutters for rock formation drilling are known to persons having ordinary skill in the art. These known disc bits range from having two or three large disc cutters, which are aimed to compete with the three-cone type roller cone bits, to having many smaller rolling disc assemblies on the face of the bit.
FIG. 1 is an elevational view of a conventional disc bit 100 in accordance with one example of the prior art. The conventional disc bit 100 includes a sub 110, a first blade 120, and a second blade 130. The sub 110 includes an upper threaded end 112 at one end and a lower bifurcated pivotal end 114 at the opposing end. The sub 110 is in the form of a hollow cylindrical body, but can be shaped differently in other embodiments. The first blade 120 is pivotally coupled to the lower bifurcated pivotal end 114 and is shown in a cutting position 105, which is oriented substantially perpendicular to the length of the sub 110. Similarly, the second blade 130 is pivotally coupled to the lower bifurcated pivotal end 114 and also is shown in the cutting position 105, which is oriented substantially perpendicular to the length of the sub 110 and substantially opposite of the first blade 120. When the first and second blades 120, 130 are positioned in a non-cutting position (not shown), the blades 120, 130 are positioned substantially axially to the sub 110 and positioned substantially below the lower bifurcated pivotal end 114. The blades 120, 130 are pivoted around a portion of the lower bifurcated pivotal end 114 to move one or more of the blades 120, 130 between the cutting position 105 and the non-cutting position.
Each of the blades 120, 130 includes radially offset larger and smaller semi-circular body portions 141, 146. The larger body portion 141 is positioned adjacent the lower bifurcated pivotal end 114, while the smaller body portion 146 extends from the end of the larger body portion 141 to a distance further away from the lower bifurcated pivotal end 114. The smaller body portion 146 includes one or more first cutters 147, each of which are positioned within a corresponding recess 148 formed within the undersurface, or a leading edge 122, of each blade 120, 130. The first cutters 147 are mounted into the recesses 148 using an axle (not shown) extending through the first cutter 147. The first cutters 147 are disk-shaped and are typically uniformly fabricated from tungsten carbide. Each first cutter 147 includes a tapered surface 149 formed radially around the circumference of the first cutter 147. This tapered surface 149 forms a cutting edge 150 for the first cutter 147. The larger body portion 141 includes one or more second cutters 142, each of which are positioned within a corresponding recess 143 formed within the undersurface, or the leading edge 122, of each blade 120, 130. The second cutters 142 are mounted into the recesses 143 using an axle (not shown) extending through the second cutter 142. The second cutters 142 are disk-shaped and are typically uniformly fabricated from tungsten carbide. Each second cutter 142 includes a tapered surface 144 formed radially around the circumference of the second cutter 142. This tapered surface 144 forms a cutting edge 145 for the second cutter 142. Each of the blades 120, 130 also includes one or more cutting inserts 155, typically formed from tungsten carbide, coupled within a corresponding circular recess 156 formed along a trailing edge 124 of each blade 120, 130. Each insert 155 is of a generally elongated cylindrical configuration which protrudes from the trailing edge 124 in order to cut into the formation when the blades 120, 130 are rotated. The cutting inserts 155 are most useful in the event of formation hole collapse, hole sloughing or hole swelling. In operation, the first and second cutters 142, 147 freely rotate around the axle so that a fresh corresponding tapered surface 144, 149 is exposable for cutting the rock formation.
FIG. 2 is a perspective view of a conventional disc bit 200 or head of a conventional shaft in accordance with another example of the prior art. The head 200 includes a generally circular bit body 205, which is adapted to be coupled to a drilling or tunneling machine (not shown) to be rotated and pushed or pulled through a rock or earthen formation to form a wellbore.
A plurality of saddle members 220 are secured to the bit body 205 at various selected locations. A cutter shell or sleeve 230 is carried for rotation by a journal member (not shown), an end of which is secured to and supported by the saddle member 220. Methods of securing journal members to saddle members 220 are known to persons having ordinary skill in the art. A plurality of disc-type cutters 250 are coupled to the face of the bit body 205 using the saddle members 220. The disc-type cutters 250 include a raised, annular kerf ring 255 and are releasably secured to each cutter sleeve or shell 230. As the bit body 205 is rotated and pushed or pulled through the formation, the cutters 250 and the kerf rings 255 engage the formation, scoring it in generally circular patterns and causing the fracture of large cuttings or fragments of rock from the formation. The cuttings (not shown) removed by disc-type cutters 250 are removed with less energy per volume of rock fractured and produce larger cuttings, which are easier to remove from the wellbore as boring progresses.
Currently disc bits are at best used in only an extremely small segment of the overall market for oilfield or blast hole mining drilling. Disc cutters are quite successful though in large diameter tunneling, or raise boring, machines. The historical failure of disc cutters in oilfield applications has been their reliance on relative small diameter axles running through the center of steel or tungsten carbide rolling disc cutters. The axles are prone to breakage from weight-on-bit and rapid wear in the abrasive drilling environment. If the axle and/or the disc interface with the axle is lubricated and sealed, then each disc assembly requires a lubrication and compensation system (not shown) which rapidly consumes the available “real estate” in the bit body. In addition, the steel disc cutters, and even those made of tungsten carbide, are prone to rapid wear of the cutting edge.
In spite of all of the above drawbacks, disc bits continue to attract attention because they offer an entirely different rock failure mechanism than the crushing/scraping of conventional roller cone bits or the shear cutting of conventional PDC bits. Disc bits allow for high point loading on the cutters and fail the rock through a slicing/plowing/spalling mechanism. In many formations, this cutting mechanism can produce very high rates of penetration at relatively low torque levels.
What is needed is a disc cutter and bit design approach that offers the advantages of the high point loading slicing/plowing/spalling available from disc cutters while overcoming the small axle and/or the rapid disc wear drawbacks of disc bits previously in the art.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.